fixup! Introduce fsck options
fsck: support demoting errors to warnings

There are some legacy repositories out there which display non-fatal
errors with fsck. Let's make fsck useful with such repositories by
allowing users to ignore known issues, or at least demote those issues
to mere warnings (e.g. when a very early, hand-crafted commit has two
authors instead of one).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! Document the new receive.fsck.* options.
fixup! fsck: handle multiple authors in commits specially